JP2014105644A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine, capable of surely reducing and purifying NOx.SOLUTION: In an exhaust emission control device of an internal combustion engine, an ammonia absorption catalyst 27, which is formed of zeolite having a Bronsted acid point and detaches absorbed ammonia when the temperature of the catalyst is a predetermined temperature (400°C) or more, is disposed at the upstream of a selective reduction type catalyst 28, which is formed of zeolite having a Lewis acid point, and at the downstream of a urea water injector 36. The ammonia absorption catalyst 27 and the selective reduction type catalyst 28 are disposed in a casing 30.

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to the structure of an exhaust gas purification apparatus.

In diesel engines and lean-burn gasoline engines, the amount of nitrogen oxide (NOx) discharged into the exhaust gas increases because the mixture of fuel and air contains a large amount of oxygen.
Therefore, conventionally, in a diesel engine or a lean burn gasoline engine, a selective reduction catalyst, a NOx trap catalyst, or the like is provided in the exhaust passage. Particularly in a diesel engine, the amount of NOx emission increases during high rotation speed and high load operation. Therefore, ammonia water (NH3) generated by adding urea water to the exhaust gas in the exhaust passage and hydrolyzing the urea water A selective reduction catalyst device (SCR system) that reduces and purifies NOx is used.

  In such a selective reduction catalyst using ammonia as a reducing agent, as disclosed in Patent Document 1, nitrogen monoxide is supplied into the exhaust passage upstream of the nitrogen oxide reduction catalytic converter (ammonia selective reduction catalyst). Nitrogen oxide generating means is provided. Further, an ammonia generating catalytic converter for generating ammonia from nitrogen monoxide supplied in the exhaust passage is disposed downstream of the nitrogen monoxide generating means, and ammonia is supplied to the nitrogen oxide reducing catalytic converter to There is a device to reduce and purify.

Special table 2003-500590 gazette

In the exhaust gas purification device of Patent Document 1, ammonia is generated from nitrogen monoxide by an ammonia generating catalytic converter, and ammonia is supplied to the nitrogen oxide reducing catalytic converter.
Thus, when nitric oxide is supplied from the nitric oxide generating means to the ammonia generating catalytic converter, ammonia is generated by the ammonia generating catalytic converter, and NOx is reduced and purified by the nitrogen oxide reducing catalytic converter, It takes time from supply of nitrogen to generation of ammonia, and further time is required to reduce and purify NOx.

Therefore, when the internal combustion engine has a large amount of NOx discharged from the internal combustion engine and has a high rotational speed and a high load, it takes a long time from the generation of ammonia to the reduction and purification of NOx. It becomes difficult to reduce and purify with an oxide reduction catalytic converter.
The present invention has been made to solve such problems, and an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can reliably reduce and purify NOx.

In order to achieve the above object, in the exhaust gas purification apparatus for an internal combustion engine according to claim 1, urea water is supplied, and nitrogen oxides in the exhaust gas of the internal combustion engine are generated by ammonia generated by hydrolysis of the urea water. An exhaust purification device for an internal combustion engine that purifies, an ammonia adsorption catalyst that adsorbs and desorbs the ammonia in a predetermined temperature region, a urea water supply means that supplies the urea water upstream of the ammonia adsorption catalyst, and a predetermined temperature region A selective reduction catalyst that adsorbs and desorbs the ammonia and reduces and purifies the nitrogen oxides with the ammonia, and the ammonia adsorption catalyst is disposed upstream of the selective reduction catalyst.
The ammonia desorption temperature of the ammonia adsorption catalyst is higher than the ammonia desorption temperature of the selective catalytic reduction catalyst.

The exhaust gas purification apparatus for an internal combustion engine according to claim 2 is characterized in that, in claim 1, the ammonia adsorption catalyst is a zeolite catalyst having a Bronsted acid point.
The exhaust gas purification apparatus for an internal combustion engine according to claim 3 is characterized in that, in claim 1 or 2, a catalyst capacity of the ammonia adsorption catalyst is equal to or less than a catalyst capacity of the selective reduction catalyst.

  The exhaust gas purification apparatus for an internal combustion engine according to claim 4 is characterized in that, in any one of claims 1 to 3, the selective reduction catalyst is a zeolite catalyst containing a metal ion and having a Lewis acid point. To do.

  According to the first aspect of the present invention, the ammonia adsorption catalyst having an ammonia desorption temperature higher than the ammonia desorption temperature of the selective reduction catalyst is disposed upstream of the selective reduction catalyst, so that it is discharged from the internal combustion engine. Ammonia adsorbed catalyst adsorbs ammonia generated by hydrolyzing urea water during low-load operation of an internal combustion engine with relatively low nitrogen oxides, and the exhaust gas becomes high temperature during high-load operation of the internal combustion engine. When the temperature becomes high, ammonia adsorbed on the ammonia adsorption catalyst can be desorbed, and ammonia can be supplied to the selective reduction catalyst.

  Thus, for example, when the vehicle suddenly accelerates and the internal combustion engine is operated at a high load and the amount of nitrogen oxides discharged from the internal combustion engine increases, the exhaust gas becomes hot and the ammonia adsorption catalyst becomes hot. Ammonia adsorbed by the adsorption catalyst is desorbed, and ammonia can be supplied to the selective catalytic reduction catalyst in addition to ammonia generated from the urea water supplied from the urea water supply means. That is, in order to cope with an increase in nitrogen oxide emission due to high load operation, the urea water supply amount is increased, the ammonia supply amount is increased, and the nitrogen oxide is reduced by the ammonia in the selective catalytic reduction catalyst. Until the reduction and purification, the selective reduction catalyst can be used for reduction and purification using ammonia desorbed from the ammonia adsorption catalyst.

Therefore, even if the discharge amount of nitrogen oxides discharged from the internal combustion engine increases due to high load operation of the internal combustion engine, the nitrogen oxides can be reliably reduced and purified by the selective reduction catalyst.
Further, in addition to ammonia generated by hydrolysis of urea water supplied from the urea water supply means, ammonia can be desorbed from the ammonia adsorption catalyst, and ammonia can be supplied to the selective reduction catalyst. Considering the amount of urea water that is discharged without being hydrolyzed when the flow rate of exhaust gas is increased during high load operation of the internal combustion engine, it is not necessary to supply urea water, reducing the consumption of urea water, and The amount of ammonia discharged outside the vehicle can be reduced.

According to the invention of claim 2, the ammonia adsorption catalyst is a zeolite catalyst having a Bronsted acid point.
The zeolite catalyst having a Brested acid point has a temperature at which desorption of ammonia is about 400 ° C. or higher, relatively little nitrogen oxides are discharged from the internal combustion engine, and the exhaust gas is low in temperature (less than 400 ° C.). Ammonia can be adsorbed during low-load operation of the internal combustion engine, etc., and ammonia can be desorbed during high-load operation of the internal combustion engine where the exhaust gas becomes high temperature (400 ° C. or higher).

Therefore, the exhaust gas becomes high temperature (400 ° C. or higher), and the amount of nitrogen oxide emission increases. During the high load operation of the internal combustion engine, ammonia can be desorbed reliably, and ammonia can be supplied to the selective catalyst. Can reduce and purify nitrogen oxides.
According to the invention of claim 3, since the catalyst capacity of the ammonia adsorption catalyst is set to be equal to or less than the catalyst capacity of the selective reduction catalyst, even if all of the ammonia adsorbed on the ammonia adsorption catalyst is desorbed, Ammonia can be used for reduction and purification of nitrogen oxides with a selective reduction catalyst.

Therefore, the amount of ammonia adsorbed on the ammonia adsorption catalyst does not exceed the amount of ammonia used for the purification of nitrogen oxides in the selective reduction catalyst, so a part of the desorbed ammonia is selectively reduced catalyst. It is possible to prevent the nitrogen oxides from being discharged outside the vehicle without being used for the reduction and purification of nitrogen oxides.
According to the invention of claim 4, the selective catalytic reduction catalyst is a zeolite catalyst containing a metal ion and having a Lewis acid point.

  The zeolite catalyst having a Lewis acid point has a temperature at which desorption of ammonia starts from about 200 ° C. to 300 ° C., so that ammonia is adsorbed and desorbed during low to medium load operation of the internal combustion engine. The nitrogen oxides discharged from the gas can be reduced and purified suitably.

1 is a schematic configuration diagram of an engine to which an exhaust gas purification apparatus for an internal combustion engine according to the present invention is applied.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an engine 1 to which an exhaust gas purification apparatus for an internal combustion engine is applied.
As shown in FIG. 1, an engine (internal combustion engine) 1 is a multi-cylinder direct injection internal combustion engine (for example, a common rail diesel engine). Specifically, high pressure fuel accumulated in the common rail is used as fuel for each cylinder. The fuel nozzle is supplied to the injection nozzle 2 and can be injected from the fuel injection nozzle 2 into the combustion chamber 3 of each cylinder at an arbitrary injection timing and injection amount.

Each cylinder of the engine 1 is provided with a piston 4 that can slide up and down. The piston 4 is connected to the crankshaft 6 via a connecting rod 5. A flywheel is provided at one end of the crankshaft 6.
An intake port 7 and an exhaust port 8 are communicated with the combustion chamber 3.
The intake port 7 is provided with an intake valve 9 that communicates and shuts off the combustion chamber 3 and the intake port 7. Further, the exhaust port 8 is provided with an exhaust valve 10 for performing communication between the combustion chamber 3 and the exhaust port 8 and shutting off.

An intake manifold 11 that distributes the sucked air to each cylinder is provided upstream of the intake port 7 so as to communicate therewith. Further, an exhaust manifold 12 that collects exhaust gas discharged from each cylinder is provided downstream of the exhaust port 8.
The intake manifold 11 upstream of the branch for distributing the intake air to each cylinder of the intake manifold 11 is provided with an oxygen concentration sensor 13 for detecting the oxygen concentration so that the sensor portion projects into the intake manifold 11. . Further, an intake air temperature sensor 14 for detecting the temperature of intake air taken into the combustion chamber 3 is provided downstream of the air-fuel ratio sensor 13 so as to protrude into the intake manifold 11.

  The intake manifold 11 and the exhaust manifold 12 are provided with an EGR passage 15 for returning a part of the exhaust gas to the intake air, that is, for introducing EGR gas into the intake air so as to communicate with each other. The EGR passage 15 is connected to an intake manifold 11 upstream of the oxygen concentration sensor 13 via an EGR valve 16 that adjusts the amount of exhaust gas returning to the intake air, that is, the flow rate of EGR gas. The EGR passage 15 is provided with an EGR cooler 17 that cools the exhaust gas introduced into the intake manifold 11.

  Upstream of the intake manifold 11, an air cleaner 18 that removes fresh dust sucked from the uppermost stream, a compressor housing (not shown) of a turbocharger 19 that compresses the sucked fresh air using the energy of the exhaust, An intercooler 20 that cools the compressed and heated fresh air is connected to the intake manifold 11 via an intake pipe 21. An electronically controlled throttle valve 22 for adjusting the flow rate of EGR gas introduced from the EGR passage 15 is disposed between the intake manifold 11 and the intake pipe 21. The electronically controlled throttle valve 22 is provided with a throttle position sensor 23 for detecting the opening degree of the throttle valve.

The intake pipe 21 between the air cleaner 18 of the intake pipe 21 and the compressor housing of the turbocharger 19 is provided with an intake temperature sensor 38 that detects the temperature of the intake air so as to protrude into the intake pipe 21. Yes.
Downstream of the exhaust manifold 12, a turbine housing (not shown) for introducing exhaust gas into the turbocharger 19 and an exhaust pipe 24 are provided so as to communicate with each other.

  In the exhaust pipe 24, an oxidation catalyst 25 that oxidizes oxidizable components in the exhaust gas in order from the upstream, a diesel particulate filter 26 that collects and burns particulate matter mainly composed of graphite in the exhaust gas, and ammonia Is temporarily adsorbed, and when the exhaust gas temperature becomes equal to or higher than a predetermined temperature (400 ° C.), the ammonia adsorption catalyst 27 that desorbs the adsorbed ammonia and the nitrogen oxide (hereinafter referred to as NOx) in the exhaust gas are reduced and purified using ammonia. The selective reduction catalyst 28 is provided so as to communicate with it. Note that the oxidation catalyst 25 and the diesel particulate filter 26 are disposed in a casing 29. Further, the ammonia adsorption catalyst 27 and the selective reduction catalyst 28 are disposed in the casing 30.

  The ammonia adsorption catalyst 27 is made of a zeolite that does not contain metal ions (for example, Cu and Fe), that is, a zeolite having a Bronsted acid point. The catalyst capacity of the ammonia adsorption catalyst 27 is set to be equal to or less than the catalyst capacity of the selective reduction catalyst 28. When the temperature of the catalyst is lower than a predetermined temperature (400 ° C.), the ammonia adsorption catalyst 27 adsorbs ammonia generated by hydrolysis of urea water injected from the urea water injector 36 described later, and the temperature of the catalyst is a predetermined temperature. If it becomes above, it has the characteristic of desorbing the adsorbed ammonia. That is, when the exhaust gas temperature becomes equal to or higher than the predetermined temperature and the temperature of the ammonia adsorption catalyst 27 becomes equal to or higher than the predetermined temperature, the ammonia adsorption catalyst 27 desorbs ammonia and supplies ammonia to the downstream selective reduction catalyst 28.

  The selective reduction catalyst 28 is composed of a zeolite containing metal ions (for example, Cu or Fe), that is, a zeolite having a Lewis acid point. The selective reduction catalyst 28 has a function of adsorbing and desorbing ammonia supplied to the selective reduction catalyst 28 in addition to the function of reducing and purifying NOx. The selective reduction catalyst 28 is formed by ammonia desorbed from the ammonia adsorption catalyst 27, ammonia generated by hydrolysis of urea water injected from the urea water injector 36, or ammonia adsorbed on the selective reduction catalyst 28. NOx in exhaust gas is reduced and purified. The selective catalytic reduction catalyst 28 starts desorption of ammonia when the temperature of the catalyst is about 200 ° C. to 300 ° C.

Between the turbocharger 19 of the exhaust pipe 24 and the casing 29, an oxygen concentration sensor 31 for detecting the oxygen concentration in the exhaust gas is provided so as to protrude into the exhaust pipe 24.
An exhaust gas temperature sensor 32 that detects the temperature of the exhaust gas flowing into the oxidation catalyst 25 is provided upstream of the oxidation catalyst 25 in the casing 29 so as to protrude into the casing 29. An exhaust temperature sensor 33 that detects the temperature of the exhaust gas flowing out from the oxidation catalyst 25 is provided between the oxidation catalyst 25 of the casing 29 and the diesel particulate filter 26 so as to protrude into the casing 29. An exhaust gas temperature sensor 34 that detects the temperature of the exhaust gas flowing out from the diesel particulate filter 26 is provided downstream of the diesel particulate filter 26 in the casing 29 so as to project into the casing 29.

  Between the diesel particulate filter 26 and the ammonia adsorption catalyst 27 in the exhaust pipe 24, that is, between the casing 29 and the casing 30 of the exhaust pipe 24, a NOx sensor 35 that detects the concentration of NOx in the exhaust gas is provided in the exhaust pipe 24. It is provided so as to protrude inside. A urea water injector (urea water supply means) 36 is provided between the NOx sensor 35 of the exhaust pipe 24 and the casing 30 so as to protrude into the exhaust pipe 24.

  The urea water injector 36 injects urea water for supplying ammonia to the ammonia adsorption catalyst 27 and the selective reduction catalyst 28 into the exhaust pipe 24. The urea water is supplied from a urea water tank (not shown) provided outside. The distance from the urea water injector 36 to the ammonia adsorption catalyst 27 is set to be longer than the distance necessary for the urea water injected from the urea water injector 36 to be hydrolyzed to generate ammonia. Therefore, the urea water injected from the urea water injector 36 undergoes hydrolysis and generates ammonia before reaching the ammonia adsorption catalyst 27.

A NOx sensor 37 that detects the concentration of NOx in the exhaust gas flowing out from the selective reduction catalyst 28 protrudes into the exhaust pipe 24 downstream of the selective reduction catalyst 28 of the exhaust pipe 24, that is, downstream of the casing 30. Is provided.
The fuel injection nozzle 2, oxygen concentration sensors 13, 31, intake air temperature sensors 14, 38, EGR valve 16, electronic control throttle valve 22, throttle position sensor 23, exhaust temperature sensors 32, 33, 34, NOx sensors 35, 37 Various devices such as an urea water injector 36, various sensors for detecting the operating state of the engine 1 and an accelerator position sensor for detecting the degree of operation of an accelerator pedal operated by a driver of a vehicle on which the engine 1 is mounted are the engine 1 Engine control unit comprising an input / output device, a storage device (ROM, RAM, nonvolatile RAM, etc.), a timer, a central processing unit (CPU), etc. 50 is electrically connected. The engine control unit 50 controls the operation of the engine 1 by controlling the operation of various devices based on information from various sensors.

  When the engine control unit 50 determines that the NOx emission amount is larger than the operating state of the engine 1 or the NOx concentration detected by the NOx sensor 35, the supply amount of ammonia supplied to the ammonia adsorption catalyst 27 and the selective reduction catalyst 28. The amount of urea water injected from the urea water injector 36 is increased so as to increase. When the engine control unit 50 determines that the NOx emission amount is smaller than the operating state of the engine 1 or the NOx concentration detected by the NOx sensor 35, the engine control unit 50 uses the selective reduction type to adsorb ammonia to the ammonia adsorption catalyst 27. The amount of urea water injected from the urea injector 36 is adjusted so that the amount of ammonia required to purify NOx by the catalyst 28 is equal to the amount of ammonia added to the ammonia adsorption catalyst 27. The engine control unit 50 constantly monitors the amount of ammonia adsorbed on the ammonia adsorption catalyst 27, and when the ammonia amount adsorbed on the ammonia adsorption catalyst 27 becomes the limit ammonia amount that can be adsorbed on the ammonia adsorption catalyst 27, the selective reduction type. The amount of urea water injected from the urea injector 36 is adjusted so that the amount of ammonia required for purifying NOx by the catalyst 28 is obtained.

  In the exhaust gas purification apparatus for an internal combustion engine according to the present invention configured as described above, the ammonia adsorption is composed of zeolite having a Bronsted acid point and desorbs the adsorbed ammonia when the temperature of the catalyst reaches a predetermined temperature (400 ° C.) or higher. The catalyst 27 is disposed upstream of the selective reduction catalyst 28 made of zeolite having a Lewis acid point and downstream of the urea water injector 36. An ammonia adsorption catalyst 27 and a selective reduction catalyst 28 are disposed in the casing 30.

  Therefore, it is generated by hydrolyzing the urea water injected from the urea water injector 36 at the time of low load operation of the engine 1 where the NOx discharged from the engine 1 is relatively small and the exhaust gas is less than a predetermined temperature (400 ° C.). The adsorbed ammonia is adsorbed on the ammonia adsorption catalyst 27, and when the engine 1 is operated at a high load, the exhaust gas becomes a predetermined temperature or higher, and when the ammonia adsorption catalyst 27 exceeds the predetermined temperature, the ammonia adsorbed on the ammonia adsorption catalyst 27 is desorbed, Ammonia can be supplied to the selective catalytic reduction catalyst.

  For example, when the vehicle suddenly accelerates and the engine 1 becomes a high-load operation and the amount of NOx discharged from the engine 1 increases, the exhaust gas reaches a predetermined temperature (400 ° C.) or higher, and the ammonia adsorption catalyst 27 When the temperature exceeds the temperature, the ammonia adsorbed on the ammonia adsorption catalyst 27 is desorbed, and ammonia can be supplied to the selective catalytic reduction catalyst 28 in addition to the ammonia generated from the urea water injected from the urea water injector 36. Then, the urea water injection amount is increased in order to cope with the increase in the NOx emission amount in the high load operation of the engine 1, the urea water is hydrolyzed to become ammonia, and the selective reduction catalyst 28 uses the ammonia. Until the NOx is reduced and purified, NOx discharged from the engine 1 can be reduced and purified by the selective reduction catalyst 28 using ammonia desorbed from the ammonia adsorption catalyst 27.

Therefore, even if the amount of NOx discharged from the engine 1 increases due to high load operation of the engine 1 or the like, the selective reduction catalyst 28 can reliably reduce and purify NOx.
Further, in addition to ammonia generated by hydrolysis of urea water injected from the urea water injector, ammonia can be desorbed from the ammonia adsorption catalyst 27 and supplied to the selective reduction catalyst 28. Considering the amount of urea water discharged without being hydrolyzed when the flow rate of the exhaust gas is increased during high load operation of the engine 1, it is not necessary to supply the amount of urea water, thus reducing the consumption of urea water, In addition, the amount of ammonia discharged out of the vehicle can be reduced.

Further, since the catalyst capacity of the ammonia adsorption catalyst 27 is set to be equal to or less than the catalyst capacity of the selective reduction catalyst 28, even if all ammonia adsorbed on the ammonia adsorption catalyst 27 is desorbed, all desorbed ammonia is selected. The reduction catalyst 28 can be used for NOx reduction purification.
Therefore, since the amount of ammonia adsorbed on the ammonia adsorption catalyst 27 does not exceed the amount of ammonia used for purification of NOx nitrogen oxides by the selective reduction catalyst 28, a part of the desorbed ammonia is selected. It is possible to prevent the reduction catalyst 28 from being discharged outside the vehicle without being used for NOx reduction purification.

  Further, the ammonia adsorption catalyst 27 and the selective reduction catalyst 28 are disposed in the casing 30 so that the ammonia adsorption catalyst 27 is located upstream of the selective reduction catalyst 28, and therefore, upstream of the selective reduction catalyst 28. The dispersibility of ammonia can be improved by the rectifying effect of the tandem catalyst by disposing the ammonia adsorption catalyst 27, and the NOx purification efficiency in the selective reduction catalyst 28 can be improved.

Furthermore, since the distance from the urea water injector 36 to the ammonia adsorption catalyst 27 is set to be longer than the distance necessary for the urea water injected from the urea water injector 36 to be hydrolyzed to generate ammonia, it is ensured that the urea Ammonia can be generated from water and supplied to the ammonia adsorption catalyst 27 and the selective reduction catalyst 28.
Therefore, it is possible to prevent the urea water from being discharged outside the vehicle.

  Further, the selective catalytic reduction catalyst 28 is a zeolite catalyst containing a metal ion and having a Lewis acid point, and the zeolite catalyst having a Lewis acid point has a temperature at which desorption of ammonia starts from about 200 ° C. to 300 ° C. Ammonia is adsorbed and desorbed during low load to medium load operation of the engine 1 and the NOx discharged from the engine 1 can be reduced and purified appropriately.

Although the description of the embodiment of the invention is finished as above, the embodiment of the present invention is not limited to the above embodiment.
In the above embodiment, the engine 1 is a common rail type diesel engine. However, the present invention is not limited to this, and a lean combustion gasoline engine having an exhaust system that has a relatively large NOx emission amount and a selective reduction catalyst is mounted. Needless to say, this is applicable.

1 engine (internal combustion engine)
27 Ammonia adsorption catalyst 28 Selective reduction catalyst 36 Urea water injector (urea water supply means)

Claims (4)

An exhaust purification device for an internal combustion engine that supplies urea water to purify nitrogen oxides in exhaust gas of the internal combustion engine with ammonia generated by hydrolysis of the urea water,
An ammonia adsorption catalyst that adsorbs and desorbs the ammonia in a predetermined temperature range;
Urea water supply means for supplying the urea water upstream of the ammonia adsorption catalyst;
A selective reduction catalyst that adsorbs and desorbs the ammonia in a predetermined temperature range and reduces and purifies the nitrogen oxides with the ammonia; and
The ammonia adsorption catalyst is disposed upstream of the selective reduction catalyst,
An exhaust purification device for an internal combustion engine, wherein an ammonia desorption temperature of the ammonia adsorption catalyst is higher than the ammonia desorption temperature of the selective reduction catalyst.   The exhaust purification apparatus for an internal combustion engine according to claim 1, wherein the ammonia adsorption catalyst is a zeolite catalyst having a Bronsted acid point.   3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein a catalyst capacity of the ammonia adsorption catalyst is equal to or less than a catalyst capacity of the selective reduction catalyst.   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the selective reduction catalyst is a zeolite catalyst containing metal ions and having a Lewis acid point.

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